Soil and Valley Effects in Bridge Foundation Motion

The paper refers to the seismic response of a bridge, founded on piles in a valley characterized by sharp impedance contrast with the underlying soil and by steep lateral boundaries. The bridge is a real-scale experiment as during many earthquake records have been obtained of: (a) the ground motions at the base and the surface of the valley, (b) the seismic response of a bridge pier, and (c) the seismic bending strains developed on its pile foundation. As one-dimensional analyses proved inadequate to capture the two-dimensional valley effects and to predict the recorded ground surface motions, two-dimensional seismic response analyses have been performed using the finite-element method along with the “effective seismic excitation” technique. The available records testify the successful prediction of the free-field motion. The model is then extended to incorporate the pile foundation and the superstructure. In addition, the successful estimation of the free-field field motion in the vicinity of the pile helps the implementation of an improved analytical model for computing the kinematic bending strain. Despite the simplifications in the numerical modeling and the limitations of the analytical solution, results are in agreement with the records. The importance of assessing realistically site specific ground motions for bridge foundation design is demonstrated